Thermoplastic resin microporous membranes have been widely used, for example, as a material for separation, selective permeation, and isolation of substances: e.g., battery separators used in a lithium ion secondary battery, nickel-hydrogen battery, nickel-cadmium battery, and polymer battery; separators for an electric double layer capacitor; various filters such as a reverse osmosis filtration membrane, ultrafiltration membrane, and microfiltration membrane; moisture-permeable waterproof clothing; and medical materials. In particular, polyolefin microporous membranes have been suitably used as a lithium ion secondary battery separator, because they are not only characterized by having excellent electrical insulating properties, having ion permeability due to electrolyte impregnation, and having excellent electrolyte resistance and oxidation resistance, but also have such a pore-blocking effect that excessive temperature rise is suppressed by blocking a current at a temperature of about 120 to 150° C. in abnormal temperature rise in a battery. However, when the temperature continues to rise for some reason even after pore blocking, membrane rupture can occur at a certain temperature due to decrease in viscosity of molten olefin constituting the membrane and shrinkage of the membrane. In addition, when left at a constant high temperature, membrane rupture can occur after the lapse of a certain time due to decrease in viscosity of molten polyolefin and shrinkage of the membrane. This phenomenon is not a phenomenon that occurs only when polyolefin is used, and also when other thermoplastic resins are used, this phenomenon is unavoidable at or higher than the melting point of the resin constituting the porous membrane.
In particular, separators for a lithium ion battery are highly responsible for battery properties, battery productivity, and battery safety, and required to have excellent mechanical properties, heat resistance, permeability, dimensional stability, pore-blocking properties (shutdown properties), melt rupture properties (meltdown properties), and the like. Accordingly, various studies to improve heat resistance have been conducted until now. Fluororesins which have both heat resistance and oxidation resistance have been suitably used as a heat resistant resin layer.
Further, to increase battery capacity, it is expected that membranes will become thinner and thinner in order to increase the area not only of electrodes but also of a separator that can be loaded into a container. When a porous film becomes thinner, it tends to be deformed in the planar direction, and accordingly a heat resistant resin layer may be peeled off during processing of a battery separator, a slitting process, or a battery assembly process, which makes it difficult to ensure safety.
Further, to achieve cost reduction, it is expected that the speed will be faster in a battery assembly process, and the present inventors presume that there will be a demand for even higher adhesion to withstand high-speed processing, by which troubles such as peeling-off of a heat resistant resin layer hardly occurs even in such high-speed processing. Furthermore, it is expected that there will be an increasing demand for higher processability (lower curling properties) in a battery assembly process in the future.
Patent Document 1 discloses a lithium ion secondary battery separator obtained by direct application of a polyamide-imide resin to a polyolefin porous membrane with a thickness of 25 lam to a thickness of 1 μm and immersion in water at 25° C., followed by drying.
In Patent Document 1, significant increase in air resistance was unavoidable, and, in addition, curling was large and not satisfactory.
Patent Document 2 discloses an electrolyte-supported polymer membrane obtained by immersion of a nonwoven fabric with an average thickness of 36 μm comprising aramid fibers in a dope containing a vinylidene fluoride copolymer which is a heat resistant resin, and drying.
Patent Document 3 discloses a composite porous membrane obtained by immersion of a polypropylene microporous membrane with a thickness of 25.6 μm in a dope mainly composed of polyvinylidene fluoride which is a heat resistant resin, followed by the process of a coagulation bath, washing with water, and drying.
As in Patent Document 2, in a method in which coating is performed by dipping (immersing) a nonwoven fabric comprising aramid fibers in a heat resistant resin solution, a heat resistant porous layer is formed inside and on both surfaces of the nonwoven fabric, and accordingly most of continuous pores inside the nonwoven fabric will be blocked; consequently, significant increase in air resistance is unavoidable, and besides a pore-blocking function, the most important function that determines safety of a separator, cannot be provided.
In addition, nonwoven fabrics are difficult to thin as compared to polyolefin porous membranes, and therefore are not suitable for increase in battery capacity which is expected to progress in the future.
Also in Patent Document 3, a heat resistant porous layer is similarly formed inside and on both surfaces of a polypropylene microporous membrane. As in Patent Document 2, significant increase in air resistance is unavoidable, and it is difficult to obtain a pore-blocking function.
Patent Document 4 discloses a separator having a heat resistant porous layer comprising para-aramid obtained in such a manner that, when a solution of para-aramid resin which is a heat resistant resin is applied to a polyethylene porous film with a thickness of 25 μm, the polyethylene porous film is impregnated in advance with a polar organic solvent used in the heat resistant resin solution in order to avoid significant increase in air resistance, and after the heat resistant resin solution is applied, the polyethylene porous film is made into a white opaque membrane in a thermo-hygrostat set at a temperature of 30° C. and a relative humidity of 65%, and then washed and dried.
In Patent Document 4, there is no significant increase in air resistance, but adhesion between the polyethylene porous film and the heat resistant resin is extremely low. Particularly when the thickness of the polyethylene porous film is less than 10 μm, the film tends to be deformed in the planar direction, and accordingly the heat resistant resin layer may be peeled off in a battery assembly process, which makes it difficult to ensure safety.
Patent Document 5 discloses a composite porous membrane obtained in such a manner that a propylene film is coated with a polyamide-imide resin solution and passed through an atmosphere at 25° C. and 80% RH over 30 seconds to obtain a semi-gel like porous membrane; then a polyethylene porous film with a thickness of 20 μm or 10 μm is laminated onto the semi-gel like porous membrane, immersed in an aqueous solution containing N-methyl-2-pyrrolidone (NMP), and then washed with water and dried. However, curling was not satisfactory.
Further, in Patent Document 5, there is no significant increase in air resistance, but adhesion between the polyethylene porous film and the heat resistant resin is extremely low. Similarly to Patent Document 4, particularly when the thickness of the polyethylene porous film is less than 10 μm, the heat resistant resin layer may be peeled off, which makes it difficult to ensure safety.
As described above, in a battery separator in which a heat resistant resin layer is laminated on a porous membrane based on polyolefin or the like that serves as a substrate, when the heat resistant resin is infiltrated into the porous membrane that serves as a substrate to improve the adhesion of the heat resistant resin layer, the amount of air resistance increase is large. When the infiltration of the heat resistant resin is reduced, the amount of air resistance increase can be kept small, but the adhesion of the heat resistant resin layer decreases. In particular, with separators becoming thinner, in light of the speed-up in a battery assembly process, it becomes difficult to ensure safety and productivity, the demand for which will be increasingly greater. In particular, as the thickness of the polyolefin porous membrane that serves as a substrate becomes thin, it becomes more difficult to ensure low curling properties.
In other words, there has not been a battery separator that has low curling properties and a balance between adhesion of a heat resistant resin layer and the amount of air resistance increase, in which battery separator the thickness of a polyolefin porous membrane that serves as a substrate is less than 10 μm. As the thickness of a porous membrane based on polyolefin or the like that serves as a substrate becomes thinner, it becomes more and more difficult to achieve a balance between adhesion of a heat resistant resin layer and the amount of air resistance increase.